The present invention relates to an image forming apparatus such as a printer, and specifically relates to an image forming apparatus performing image formation by transferring a toner image onto a member to be transferred.
In a conventional image forming apparatus using an electrophotographic system, a transfer means principally using a contact charging scheme adopts a control method, which is called xe2x80x9cATVC (active transfer voltage control)xe2x80x9d method, wherein a current is caused to flow through a transfer portion at a time other than the time of image transfer and an optimum transfer bias is set on the basis of current and voltage values at that time.
This control method will be described with reference to FIG. 10. FIG. 10 is an explanatory view of the conventional image forming apparatus.
Referring to FIG. 10, the image forming apparatus includes a photosensitive drum 101 as an image bearing member, a primary charging means 102, an exposure means 103, a developing means 104, a transfer means 105, an a cleaner 106.
After the photosensitive drum 101 is uniformly charged by the primary charging means (charger) 102, an electrostatic latent image is formed on the photosensitive drum 101 by performing image exposure based on an image signal by means of the exposure means 103. Thereafter, the latent image is developed with a toner by the developing means (apparatus) 104 to form a toner image, and the toner image formed on the photosensitive drum 101 is transferred onto a transfer medium P. Toner particles remaining on the photosensitive drum 101 after the transfer operation are recovered by the cleaner 106.
In FIG. 10, the transfer means 105 is of a contact charging scheme using an elastic roller which has been frequently used conventionally in the electrostatic image forming apparatus in view of advantages, such as ozone-less system and cost reduction.
However, it is difficult to suppress an irregularity in resistance of the elastic roller as the above-mentioned transfer means (hereinafter referred to as a xe2x80x9ctransfer rollerxe2x80x9d) at the time of production, and the resistance varies depending on a change in temperature and/or humidity in an ambient environment and a deterioration of the transfer roller in successive image formation.
For this, reason, a method wherein a control means capable of allowing constant current control and constant voltage control with respect to a transfer high-voltage power supply and a detection means for detecting voltage and current at the time of constant current/voltage control are used so that a transfer bias is subjected to the constant current control at the time of pre-rotation for image formation and a charge potential of the photosensitive drum 101 at that time and an optimum transfer voltage at that time with respect to a resistance of the transfer roller 105 are detected to effect a constant voltage control with the detected transfer voltage when an image is transferred, has been known. According to this method, it is possible to effect an optimum transfer operation at an once determined voltage value, irrespective of a size of the transfer medium P.
On the other hand, some proposals as to an image forming apparatus capable of setting a plurality of speeds of the image bearing member and a member to be transferred at the transfer portion have been made.
Japanese Laid-Open Patent Application (JP-A) No. Hei 09-325625 discloses a method wherein a peripheral speed of a photosensitive drum is decreased in order to realize a high resolution of a laser beam printer to increase a density of laser scanning to the photosensitive drum without increasing a rotation speed of a polygon mirror for the laser scanning. At this time, with the decrease of rotation speed of the photosensitive drum, a speed of transfer means is also decreased.
Further, JP-A Hei 08-286528 discloses a method wherein a fixing speed is effectively lowered in order to ensure sufficient gloss and color-mixing properties for images in the case of using thick paper or OHP sheet as a transfer medium to correspondingly decrease a speed of transfer means.
However, in the above-mentioned methods wherein the transfer speed is lowered, the resultant transfer bias is deviated from an optimum value to cause a problem of an occurrence of image failure. With respect to this problem, in the methods described in JP-A Hei 09-325625 and JP-A Hei 08-286528 the above-mentioned ATVC is performed at each of a plurality of different transfer speeds to set optimum transfer bias at the respective transfer speeds.
However, a long image forming time is caused by performing the ATVC at the time of pre-rotation for image forming operation. The method performing the ATVC at the respective transfer speeds when the transfer speed is changed has accompanied with a problem of a longer image forming time at a slower transfer speed. Further, even in the image forming apparatus effecting the ATVC only at a specific time for each time of printing on a predetermined number of sheets or for each elapsed time, not for each image formation operation, it is necessary to perform the ATVC operation while changing the transfer speed as occasion demands, thus resulting in such a problem that users are kept waiting for a long time due to such ATVC operations.
In order to solve the problem, JP-A Hei 09-325625 discloses that current measurement corresponding to different speeds is performed by detecting a resistance of the transfer member at the fastest process speed.
However, in the case where, on the basis of a measurement result at a speed, a transfer condition at another speed is determined, there are many uncertain factors other than a difference in speed. As a result, it becomes difficult to accurately set transfer conditions.
An object of the present invention is to provide an image forming apparatus having different image forming speeds, capable of reducing a time required for determining conditions for transfer control.
Another object of the present invention is to provide an image forming apparatus having different image forming speeds, capable of performing accurate transfer control without being affected by environmental conditions.
According to the present invention, there is provided an image forming apparatus, comprising:
image forming means for forming an image on a first image bearing member,
a transfer member, capable of contacting the first image bearing member, for transferring the image on the first image bearing member to a second image bearing member,
a power supply for applying a bias to the transfer member,
detection means for detecting a voltage value and a current value at the time of applying the bias to the transfer member,
speed change means capable of changing a moving speed of the first image bearing member,
environment detection means for detecting a temperature or a humidity, and
control means for determining a transfer voltage value at the time of image transfer on the basis of an output of the detection means at the time of applying the bias to the transfer member other than the time of image transfer,
wherein the image forming means is capable of forming an image at different speeds, and the control means determines a transfer voltage value, on the basis of an output of the detection means at a predetermined speed of the first image bearing member and an output of the environment detection means, at a speed other than the predetermined speed.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.